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Actin is a globular protein that polymerize helicaly forming actin filaments (or microfilaments), which like the other two components of the cellular cytoskeleton form a three-dimensional network inside an eukariotic cell. Actin filaments provide mechanical support for the cell, determine the cell shape, enable cell movements (through pseudopods); and participate in certain cell junctions, in cytoplasmic streaming and in contraction of the cell during cytokinesis. In muscle cells they play an essential role, along with myosin, in muscle contraction. In the cytosol, actin is predominantly bound to ATP, but can also bind to ADP. An ATP-actin complex polymerizes faster and dissociates slower than an ADP-actin complex. Actin is also one of the most highly conserved proteins, differing by no more than 5% in species as diverse as algae and humans.
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Microfilaments assembly
The globular Actin is known as G-actin, while the filamentous polymer composed of G-actin subunits (a microfilament), is called F-actin. The microfilaments are the thickest of the cytoskeleton, with only 7nm in diameter. Much like the microtubules, actin filaments are polar, with the plus (+) end elongating approximately 10 times faster than the minus (-) end. The process of actin polymerization, nucleation, starts with the association of three G-actin monomers into a trimer. ATP-actin then binds the plus (+) end, and the ATP is subsequently hydrolyzed, which reduces the binding strength between neighboring units and generally destabilizes the filament. ADP-actin dissociates from the minus end and the increase in ADP-actin stimulates the exchange of bound ADP for ATP, leading to more ATP-actin units. This rapid turnover is important for the cells movement.
The protein cofilin binds to ADP-actin units and promotes their dissociation from the minus end and prevents their reassembly. The protein profilin reverses this effect by stimulating the exchange of bound ADP for ATP. In addition, ATP-actin units bound to profilin will dissociate from cofilin and are then free to polymerize. Another important component in filament production is the Arp2/3 proteins, which serve as sites for nucleation, stimulating the formation of G-actin trimers. All of these three proteins are regulated by cell signaling mechanism.
Organization
Actin filaments are assembled in two general types of structures: bundles and networks. Actin-binding proteins dictate the formation of either structure since they cross-link actin filaments. Actin filaments have the appearance of a double-stranded helix.
Bundles
There are two types of actin bundles: parallel and contractile bundles. In parallel bundles, the filaments are spaced 14nm apart by the actin-bundling proteins fimbrin. Parallel bundles are responsible for the supporting a cells microvilli. In vertebrates, the actin-bundling protein villin is almost entirely found in the microvilli of intestinal cells.
Muscular Contraction
Main article: Muscle contraction
Together with myosin filaments actin it forms Actomyosin, which provides the mechanism for muscle contraction. Actin uses ATP for energy. The ATP allows, through hydrolysis, the myosin head to extend up and bind with the actin filament. The myosin head then releases after moving the actin filament in a relaxing or contracting movement by usage of ADP.
In contractile bundles, the actin-bundling protein actinin separates each filament by 40nm. This increase in distance allows the motor protein myosin to interact with the filament, enabling deformation or contraction. In the first case, one end of myosin is bound to the plasma membrane while the other end walks towards the plus end of the actin filament. This pulls the membrane into a different shape relative to the cell cortex. For contraction, the myosin molecule is usually bound to two separate filaments and both ends simultaneously walk towards their filament's plus end, sliding the actin filaments over each other. This results in the shorterning, or contraction, of the actin bundle (but not the filament). This mechanism is responsible for muscle contraction and cytokinesis, the division of one cell into two.
Networks
Actin networks, along with their actin-binding protein, filamin , form the cells cortex. This underlies the plasma membrane and is responsible for the shape of the cell.
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